We hypothesize that Fas/Fas ligand-dependent cytotoxic T lymphocyte (CTL) effector mechanisms are critically important in killing of virally infected hepatocytes. This contrasts with CTL responses to allogeneic or virally infected target cells of non- hepatic origin, where perforin-dependent mechanisms play a more prominent role. The hypothesis is based on preliminary studies indicating that defects in Fas/Fas ligand-dependent immune effector mechanisms greatly prolong in vivo expression of adenovirus encoded genes in the liver. Moreover, CTL killing of hepatocyte targets appears to be largely Fas ligand-dependent and is not dramatically impaired by perforin deficiency. In the proposed studies, we will test this hypothesis by examining mechanisms involved in CTL-mediated killing of hepatocyte targets and silencing of virally encoded genes expressed in hepatocytes. We will isolate intrahepatic CTL from adenovirus infected mice and directly examine the cytolytic effector mechanisms utilized in killing adenovirus-infected hepatocytes. The adhesion and other co-stimulatory molecules that are expressed by adenovirus-specific CTL and play a role in interactions with adenovirus-infected hepatocytes will be identified. We will determine whether different CTL effector mechanisms silence adenovirally transduced foreign genes encoding cytosolic, cell surface or secreted proteins. The hypothesis, that selective modulation of Fas-dependent effector mechanisms by adenovirus encoded immunomodulatory proteins will significantly alter the duration of hepatic expression of other virally encoded genes, will be tested. These studies will provide insights into CTL effector mechanisms involved in clearance of viral infections from the liver and have the potential to direct strategies that enhance and prolong expression of liver-directed gene therapy.